Documentation

Formats an e-mail header by encoding any non-ascii characters using UTF-8
and Q-encoding, and folding lines at appropriate points. It doesn't do
more than that, so the header name and header value should be
well-formatted give or take line length and encoding. So no non-ASCII
characters within quoted-string, quoted-pair, or atom; no semantically
meaningful signs in names; no non-ASCII characters in the header name;
etcetera.